This proposal is a plan to design, build, and test a digital microcomputer-based ambulatory monitor with a large solid state data storage capacity. The ambulatory monitor will continuously measure physiological parameters on human subjects in their natural environments. In this project, we will design and build two prototypes of the ambulatory monitor, and evaluate them extensively in a number of psychophysiological data-gathering tasks in the study of fears and emotion. The ambulatory monitor will be a miniaturized battery-powered digital microcomputer containing up to eight plug-in application modules in its rugged shock-resistant and water-resistant plastic case. Initially, we will develop the basic ambulatory monitor and five types of application modules: (1) non-volatile memory for program and data storage, (2) a heart rate module, (3) a respiration module, (4) a skin conductance module, and (5) a general-purpose amplifier module with a contour follower.